Synthesis and characterization of an α-MoO3nanobelt catalyst and its application in one-step conversion of fructose to 2,5-diformylfuran

Article abstract of DOI:10.1039/d1nj02679h

In this study, α-MoO₃nanobelts were successfully synthesized by a simple, green and economic hydrothermal method and applied as a bifunctional catalyst for one-step conversion of fructose to DFF under atmospheric air. The structure of the as-prepared α-MoO₃catalyst was characterized in detail by SEM, TEM, EDS, XRD, XPS, H₂-TPR and NH₃-TPD to better understand the relationship between structure and performance. α-MoO₃nanobelts exhibited high catalytic activities for production of DFF from HMF and fructose in atmospheric air. Under optimized reaction conditions, high DFF yields of 97.2% and 78.3% were obtained by using HMF and fructose as raw materials, respectively. Furthermore, a plausible reaction pathway was proposed for the selective oxidation of HMF to DFF according to the experimental and catalyst characterization results. Importantly, α-MoO₃is a robust catalyst that can be used at least five times without obvious loss in its catalytic activity. In brief, α-MoO₃is an easily-prepared, eco-friendly, low cost and highly effective catalyst which has potential application in one-step conversion of fructose to DFF under atmospheric air.

Full text of DOI:10.1039/d1nj02679h

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Synthesis and characterization of an a-MoO3
nanobelt catalyst and its application in one-step
Cite this: DOI: 10.1039/d1nj02679h
conversion of fructose to 2,5-diformylfuran†
a
Zhenzhen Yang, ‡*ab Bangchong Zhu,‡a Yuhan He,‡a Genlei Zhang,
‡
a
Peng Cui‡a and Jianbo He
‡
In this study, a-MoO3 nanobelts were successfully synthesized by a simple, green and economic
hydrothermal method and applied as a bifunctional catalyst for one-step conversion of fructose to DFF
under atmospheric air. The structure of the as-prepared a-MoO3 catalyst was characterized in detail by
SEM, TEM, EDS, XRD, XPS, H2-TPR and NH3-TPD to better understand the relationship between
structure and performance. a-MoO3 nanobelts exhibited high catalytic activities for production of DFF
from HMF and fructose in atmospheric air. Under optimized reaction conditions, high DFF yields of
97.2% and 78.3% were obtained by using HMF and fructose as raw materials, respectively. Furthermore,
a plausible reaction pathway was proposed for the selective oxidation of HMF to DFF according to the
experimental and catalyst characterization results. Importantly, a-MoO3 is a robust catalyst that can be
used at least five times without obvious loss in its catalytic activity. In brief, a-MoO3 is an easily-
prepared, eco-friendly, low cost and highly effective catalyst which has potential application in one-step
conversion of fructose to DFF under atmospheric air.
Received 1st June 2021,
Accepted 21st July 2021
DOI: 10.1039/d1nj02679h
rsc.li/njc
a,b-unsaturated aldehyde group, which encounters a challenge.
Introduction
Many traditional oxidants including BaMnO4,9 NaOCl,10
Biomass as one kind of practical and sustainable carbon source has 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)11 and pyridinium
been regarded as a very promising alternative resource to fossil chlorochromate (PCC)12 have been investigated for selective
fuels to produce valuable chemicals.1–3 5-Hydroxymethylfurfural oxidation of HMF to DFF. However, these oxidation processes
(HMF), which is primarily obtained from biomass by dehydration suffer from low atomic utilization, equipment corrosion and
of fructose, is an ideal intermediate for synthesis of various separation difficulties. In recent years, molecular oxygen as a
chemicals via different catalytic routes. The selective catalytic green oxidant has been intensively used for preparation of DFF
oxidation of HMF is an effective pathway to produce several kinds from HMF in various heterogenous catalytic systems. Among
of furan compounds, such as 2,5-diformylfuran (DFF), 5-formyl-2- them, Mn-,13–16 Co-,17,18 V-,19,20 and Ru-21,22 based metal/metal
furancarboxylic (FFCA), 5-hydroxymethyl-2-furancarboxylic acid oxide catalysts and non-metal carbon-based catalysts23,24
(HMFCA) and 2,5-furandicarboxylic acid (FDCA).4 Among them, enable high activity and selectivity for aerobic oxidation of
DFF has attracted much attention recently due to its potential HMF to DFF. However, the large-scale production of DFF from
application in the preparation of polymers,5 heterocyclic ligands,6 HMF is still limited by the high cost of HMF, which is typically
organic conductors,7 and pharmaceuticals.8
obtained by acid-catalyzed dehydration of fructose.
DFF is generated through partial oxidation of the hydroxy-
To reduce the production cost of DFF, integration of fructose
methyl group in HMF without disturbing the more reactive dehydration with HMF oxidation in a one-pot reaction has
attracted much attention and become a research hotspot.
Combined use of an acid and a redox catalyst by stepwise
addition is a feasible strategy for one-pot conversion of fructose
to DFF. Many effective catalyst couples have been reported,
such as Amberlyst-15 and Ru/HT,25 Fe3O4-SBA-SO3H and
K-OMS-2,26 Fe3O4-RGO-SO3H and ZnFe1.65Ru0.35O4,27 and
dilute sulfuric acid and V2O5/ceramic.28 Another feasible strat-
egy is using a bifunctional catalyst which possesses both acidity
and oxidizability. 3D flower-like micro/nano Ce–Mo composite
a School of Chemistry and Chemical Engineering, Anhui Province Key Laboratory of
Advanced Catalytic Materials and Reaction Engineering, Anhui Province Key
Laboratory of Controllable Chemistry Reaction and Material Chemical
Engineering, Hefei University of Technology, Hefei, 230009, P. R. China.
E-mail: zzyang@hfut.edu.cn
b School of Materials Science and Engineering, Hefei University of Technology,
Tunxi Road 193, Hefei, 230009, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nj02679h
‡ These authors contributed equally to this work.
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oxides (f-Ce10ÀxMoxOd),29 MoO3-containing protonated nitro- reactor and kept at 140 1C for 24 h. After cooling down to room
gen doped carbon (Mo-HNC),30 sulfonated MoO3–ZrO2,31 temperature, the product was filtered, washed several times
V-g-C3N4 (H+),32 Vanadium-embedded mesoporous carbon with ethanol and then vacuum dried at 80 1C overnight.
microspheres (V-CS),33 graphene oxide,34 bifunctional carbon
nanoplatelets35 and others36–38 have been prepared and applied
Catalyst characterization
in the one-pot synthesis of DFF from fructose. However, com- Scanning electron microscopy (SEM) and energy dispersive
plicated experimental operations were needed to achieve a high X-ray spectroscopy (EDS) were performed using an S-4800
DFF yield in both combined catalyst systems and bifunctional microscope (HITACHI; Japan). Transmission electron micro-
catalyst systems. The fructose dehydration reaction was first scopy (TEM) and high-resolution TEM (HRTEM) were per-
proceeded in a N2 atmosphere to avoid the direct oxidation of formed using a JEM-2100F (JEOL; Japan) operated at 200 kV.
fructose. After fructose was completely converted to HMF, the The X-ray diffraction (XRD) spectrum was obtained using a
N2 atmosphere was switched to an O2 atmosphere to trigger an Rigaku D/Max-2500 X-ray diffractometer (Rigaku; Japan) with
HMF oxidation reaction. The reported catalytic systems which Cu Ka radiation. The X-ray photoelectron spectroscopy (XPS)
need switching of inert gas is not a real one-step method. data was collected using a PHI-5000 versa probe (Ulvac-Phi;
Additionally, in the existing catalytic systems for DFF prepara- Japan) with Al Ka radiation. The temperature programmed
tion, pure O2 which was stored in a high-pressure cylinder was desorption (TPD) of NH3 was performed on a Micromeritics
used as an oxidant, giving high requirements for equipment 2920 Autochem II Chemisorption Analyzer under a 5% NH3/Ar
and may cause security issues.
gas flow (50 mL minÀ1) at a rate of 5 1C minÀ1 up to 550 1C. The
For large-scale production, economical, eco-friendly and temperature programmed reduction (TPR) of H2 was performed
easy to operate properties should also be emphasized while on a chemisorption analyzer under a 2% H2/Ar gas flow
pursuing efficiency in direct conversion of fructose to DFF. (30 mL minÀ1) at a rate of 5 1C minÀ1 up to 900 1C.
Atmospheric air, which is an easily available, safe and eco-
Catalytic oxidation of HMF to DFF
nomic O2 source, is more attractive for use as an oxidant for
catalytic oxidation reactions. From this perspective, a bifunc- In a typical procedure, HMF (126 mg, 1 mmol), a-MoO3 (50 mg)
tional catalyst which can catalyze the reaction of HMF oxidation and DMSO (2 mL) were added into a glass tube. The reaction
to DFF without disturbing the –OH groups in the structure of was performed under atmospheric air and maintained at the
fructose under atmospheric air will be very attractive. MoO3 reaction temperature for a specific time under vigorous stirring.
which is of great interest for catalytic, optical and electroche- Finally, the reaction was quickly terminated by cooling the
mical applications39–41 is found as thus an attractive bifunc- reactor to room temperature in an ice bath, and the sample was
tional catalyst in this paper. Firstly, we successfully synthesized taken from the mixture for product analysis.
a-MoO3 nanobelts by a simple, green and economic hydrother-
One-step conversion of fructose to DFF
mal method. Then, systematic experiments proved that a-MoO3
showed excellent catalytic performance for direct conversion of In a typical procedure, fructose (180 mg), a-MoO3 (50 mg), and
fructose to DFF under atmospheric air. The yield of DFF was up DMSO (2 mL) were added into a glass tube. The reaction was
to 78.3% under the optimized reaction conditions.
performed under atmospheric air and maintained at the reac-
tion temperature for a specific time under vigorous stirring.
Finally, the reaction was quickly terminated by cooling the
reactor to room temperature in an ice bath, and the sample was
taken from the mixture for product analysis.
Experimental
Materials
Recycling and leaching tests
5-Hydroxymethylfurfural (HMF) was purchased from Beijing
HWRK Chem Co. Ltd. Fructose, sodium molybdate dehydrate,
perchloric acid, dimethyl sulfoxide (DMSO), N,N-dimethyl-
formamide (DMF), toluene, p-chlorotoluene, methyl isobutyl
keton (MIBK), CH3CN and H2SO4 were purchased from Sino-
pharm Chemical Reagent Co. Ltd. Naphthalene, MnO2, Mn2O3,
VO2 and RuO2 was purchased from Aladdin Reagent Co., Ltd.
DFF was purchased from TCI (Shanghai) Development Co. Ltd.
All reagents were used without further purification.
In a typical procedure, after being separated from the reaction
mixture by filtration, a-MoO3 was washed with H2O and alcohol
several times and dried at 80 1C overnight. Then, a-MoO3 was
reused for the next cycle under the optimum reaction condi-
tions. The possibility for Mo leaching during the oxidation
reaction was tested as follows. After the oxidation of HMF
catalyzed by a-MoO3 proceeded under optimum reaction con-
ditions for 2 h, the reactor was cooled down to room tempera-
ture by quenching with an ice bath. Then, the catalyst was
filtered using a 0.22 mm filter, and the filtrate was reheated to
Preparation of a-MoO3 nanobelts
a-MoO3 nanobelts was easily prepared according to previous 120 1C for another 6 h.
literature.42 In a typical procedure, 16 mL of perchloric acid
solution (4 mol LÀ1) was added slowly into 10 mL of sodium
Analytical method
molybdate solution (0.2 mol LÀ1) under constant stirring. After After the reaction, the mixture was diluted to 100 mL with
thoroughly mixing, the mixture was moved into a hydrothermal deionized water and filtered using PTFF filters (0.22 mm). The
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liquid products were analyzed by HPLC (Agilent 1260 Infinity)
with an instrument equipped with a refractive index (RI)
detector and Aminex HPx-87H Ion Exclusion column (7.8 mm
 300 mm) using dilute H2SO4 solution (0.004 M) as the eluent
at a flow rate of 0.6 mL minÀ1. The fructose conversion, HMF,
DFF and FFCA yields were calculated on the basis of external
standard curves constructed with authentic standards.
Results and discussion
Catalyst characterization
Fig. 2 (a) SEM, (b) TEM, (c) HRTEM and (d) SAED images of the as-
prepared a-MoO3.
The crystal structure of the as-synthesized powder sample was
characterized using an XRD technique. As shown in Fig. 1, the
strong diffraction peaks at 2y = 12.851, 23.471, 25.91, 27.411,
33.741, 35.581, 39.121, 45.91, 46.451 and 49.341 are indexed as
the (020), (110), (040), (021), (111), (041), (060), (200), (210) and
(002) crystal planes of the orthorhombic MoO3 phase (com-
monly denoted as a-MoO3, JCPDS No. 05-0508). The absence of
noticeable impurity peaks suggests the high purity of the
a-MoO3 product. The strong intensity of the reflection peaks
of (020), (040) and (060) indicated the anisotropic growth of the
a-MoO3.
The SEM and TEM images (Fig. 2a and b) showed the
a-MoO3 sample as an entirely belt-like structure with a width
of about 40 to 100 nm, and length of about several microns. The
HRTEM image (Fig. 2c) of an individual nanobelt provided
further insight into the structure of the nanomaterial, and
some short-range ordering of oxygen vacancies could be found.
The SAED pattern (Fig. 2d) recorded perpendicular to the
growth axis of the single nanobelt could be attributed to
the [010] zone axis diffraction of a-MoO3, and suggested that
the nanobelt grew along the [001] direction with the TEM
information. We used SAED to characterize different parts of
the same nanobelt, as well as different nanobelts. All electron
diffraction patterns indicated almost the same [001] orientation
of the nanobelt, implying that the a-MoO3 nanobelts are well-
crystallized single crystallites. The EDS quantitative analysis
showed the only existence of Mo and O elements, with a molar
ratio close to 1 : 3 (Fig. S1, ESI†).
H2-TPR was performed to better understand the redox
property of a-MoO3 and the result is presented in Fig. 3a.
Two intense peaks are observed obviously over the temperature
range 570–760 1C and 4760 1C. The first peak is attributed to
the reduction of Mo6+ to Mo4+, which consists of the reduction
of small crystalline MoO3 (570–670 1C) and the reduction of
bulk crystalline MoO3 (670–760 1C). The second peak located at
higher temperature (4760 1C) is assigned to the reduction of
Mo4+ to Mo0.43 The good reducibility of a-MoO3 may be in
favour of the redox process in the conversion of HMF to DFF.
Surface acid sites are crucial to the dehydration of fructose
to HMF. NH3-TPD was performed to study the surface acid sites
of a-MoO3 and the result is shown in Fig. 3b. A broad peak was
Fig. 1 XRD pattern of the as-synthesized a-MoO3.
Fig. 3 (a) H2-TPR curve and (b) NH3-TPD curve of a-MoO3.
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Table 2 The oxidation of HMF to DFF in different solvents
observed at 300 1C, indicating medium acid sites existed on the
surface of a-MoO3.
Selectivity (%)
Entry Solvent
Conversion (%) DFF yield (%) DFF FFCA
Comparison of different metal oxides for catalyzed oxidation of
HMF to DFF
1
2
3
4
5
6
7
DMSO
DMF
Toluene
100
100
97.2
95.8
70.0
75.2
42.3
14.2
10.4
97.2
95.8
91.5
93.5
2.8
4.2
8.0
6.2
76.5
The as-prepared a-MoO3 and several common metal oxides
were first used as catalysts for the HMF oxidation reaction
under atmospheric air, and the results are summarized in
Table 1. As expected, negligible HMF conversion and DFF yield
were obtained in the absence of catalyst (Table 1, entry 1). With
a-MoO3 is used as a catalyst, 100% conversion of HMF with
97.2% yield of DFF was achieved (Table 1, entry 2). Other metal
oxides including MnO2, Mn2O3, V2O5 and RuO2 have been
reported as highy effective catalysts for selective oxidation of
HMF to DFF by using oxygen as the oxidant.44,45 However, only
moderate HMF conversions (27.8–47.3%) and DFF yields (18.5–
42.2%) were obtained in the catalysis of these metal oxides
under atmosphere air (Table 1, entries 3–6). In the above HMF
oxidation reactions, FFCA was detected as the only by-product.
The experimental results indicated that the as-prepared
a-MoO3 showed the highest catalytic activity for selective oxida-
tion of HMF to DFF under atmospheric air among the metal
oxides screened.
p-Chlorotoluene 80.4
MIBK
CH3CN
H2O
55.2
30.5
24.1
76.6 20.2
46.5 50.3
43.2 55.5
Reaction conditions: HMF 0.126 g (1 mmol), a-MoO3 50 mg, solvent 2
mL, 120 1C, 8 h.
the experimental results, DMSO and DMF offered a desirable
HMF conversion as well as DFF selectivity, probably owing to
the high stability of both HMF and DFF, as well as improved
dissolution of O2 in these solvents.
Then, the effects of reaction time, temperature and amount
of catalyst on the a-MoO3-catalyzed oxidation of HMF to DFF
were studied. From Fig. 4a, both HMF conversion and DFF yield
increased gradually with the reaction time in the initial stage,
reaching the highest at 8 h. Further prolonging the reaction
time to 10 h, the yield of DFF had a minute decrease, because
DFF continued to be oxidated to FFCA and FDCA. The selectiv-
ity of DFF decreased gradually with passage of reaction time,
but still was up to 97% at 8 h, which is the most suitable
reaction time for a-MoO3-catalyzed oxidation of HMF to DFF.
From Fig. 4b, the conversion of HMF and the yield of DFF
increased gradually as the reaction temperature increased from
90 to 120 1C. Further increasing the reaction temperature to
130 1C, the yield of DFF decreased from 97% to 95%. Further-
more, the selectivity of DFF decreased gradually with the
reaction temperature increasing from 80 to 130 1C, probably
due to high temperature favouring the conversion of DFF to
FFCA. From Fig. 4c, HMF conversion and DFF yield increased
gradually with catalyst amount increase from 10 mg to 50 mg,
and reached the highest at 50 mg under the same reaction
conditions. The results indicate that there are not enough
active sites for complete HMF conversion when the amount
of catalyst is less than 50 mg. Moreover, insufficient catalyst
loading (10 mg and 20 mg) resulted in a little lower DFF
selectivity. This may be due to the formation of other insoluble
by-products such as humins, as no FFCA or FDCA was detected.
Process parameter optimization for catalytic oxidation of HMF
to DFF by a-MoO3
Solvent has a crucial effect on the HMF oxidation reaction. As
shown in Table 2, strongly polar and high-boiling solvents,
including DMSO and DMF, are desirable reaction media for
selective oxidation of HMF to DFF catalyzed by a-MoO3. High
DFF yields (97.2% and 95.8%, respectively) and selectivities
(both 100%) were obtained in DMSO and DMF (Table 2, entries
1 and 2). Low-boiling aromatic solvents, such as toluene and
p-chlorotoluene, have been reported as good solvents for HMF
oxidation to DFF. However, in this study, toluene and
p-chlorotoluene only give moderate activities of a-MoO3 with
HMF conversions of 76.5% and 80.4%, and DFF yields of 70.0%
and 75.2%, respectively (Table 2, entries 3 and 4). Other
common low-boiling solvents, including MIBK, CH3CN and
H2O were also used, giving low HMF conversions and DFF
yields (Table 2, entries 5–7). With toluene, p-chlorotoluene,
MIBK, CH3CN and H2O as solvents, other oxidation products
including FDCA and FFCA were detected as by-products. From
Leaching tests and recyclability
The long-term stability of a catalyst is one of the most impor-
tant merits of heterogeneous catalysts. The possibility for Mo
leaching during the oxidation reaction was tested as follows.
After the oxidation of HMF catalyzed by a-MoO3 proceeded
under optimum reaction conditions for 2 h, the reactor was
cooled down to room temperature by quenching with an ice
bath. Then, the catalyst was filtered using a 0.22 mm filter, and
the filtrate was reheated to 120 1C for another 6 h. From Fig. S2
(ESI†), the yield of DFF has no obvious change after removing
the catalyst. Moreover, the reaction mixture was collected and
analyzed by ICP-OES. The concentration of molybdenum ion in
Table 1 Catalytic oxidation of HMF to DFF by different metal oxides
Selectivity (%)
Entry Catalyst Conversion (%) DFF yield (%) DFF
FFCA
1
2
3
4
5
6
—
5.0
100
30.2
27.4
47.3
44.8
2.5
97.2
23.6
18.5
42.2
36.2
50
0
MoO3
MnO2
Mn2O3
VO2
97.2
78.1
67.5
89.2
80.8
2.8
3.2
1.6
5.5
3.5
RuO2
Reaction conditions: HMF 0.126 g (1 mmol), catalyst 50 mg, DMSO 2
mL, 120 1C, 8 h.
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Fig. 5 Recycled experiments at the optimal conditions for the oxidation
of HMF to DFF catalyzed by a-MoO3. Reaction conditions: HMF 0.126 g
(1 mmol), a-MoO3 50 mg, DMSO 2 mL, 120 1C, 8 h.
Reaction mechanism of HMF oxidation over a-MoO3
To discuss the reaction mechanism of HMF oxidation to DFF
over a-MoO3, the structure of fresh a-MoO3, a-MoO3 used under
atmospheric air (a-MoO3-air) and a-MoO3 used under N2 atmo-
sphere (a-MoO3-N2) were analyzed by XPS. From Fig. 6a, the O
1s spectrum of the fresh a-MoO3 can be deconvoluted into two
peaks, which are assigned to the surface lattice oxygen (symbo-
lized as Oa) with relatively low binding energies around
529–530 eV and other oxygen species (symbolized as Ob) with
binding energies around 531–531 eV, respectively. From Table
S1 (ESI†), compared to the fresh a-MoO3, a-MoO3-air shows a
Fig. 4 Effects of (a) reaction time, (b) temperature and (c) amount of
catalyst on a-MoO3-catalyzed oxidation of HMF to DFF. If not specified,
the default reaction conditions are as follows: HMF 126 mg, a-MoO3 50
mg, DMSO 2 mL, temperature 120 1C and reaction time 8 h.
the reaction mixture was measured as only 15.8 ppm, which
means that the leaching of molybdenum ion from the catalyst
is negligible during the reaction. The reusability of a-MoO3 was
also examined for selective oxidation of HMF to DFF. The solid
catalyst was recovered by centrifugation, washed thoroughly
with water and ethanol, dried in an oven and then recycled for
the next reaction. As shown in Fig. 5, a-MoO3 could be reused
for at least five runs without obvious deactivation. HMF con-
version and DFF yield both only decreased slightly after five
funs. Interestingly, the DFF selectivity almost keeps constant
with the recycling of a-MoO3. The TOF values in the five
recycling experiments were calculated as 0.86, 0.86, 0.84, 0.82
and 0.81 respectively. Additionally, SEM and EDS were used to
analyse the morphology and composition of the recycled
catalyst which had been used five times. As shown in Fig. S3
(ESI†), the morphology and composition of the recycled catalyst
both have no obvious change. The above results demonstrate
the high stability of a-MoO3 during HMF oxidation to DFF.
Fig. 6 The XPS spectra of (a) O 1s and (b) Mo 3d of a-MoO3.
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lower ratio of Oa/(Oa + Ob). Furthermore, when the reaction was
performed in a N2 atmosphere, the ratio of Oa/(Oa + Ob)
decreased obviously further after the reaction. From Fig. 6b,
in the Mo 3d spectrum of the fresh catalyst, the two peaks at
233 and 236 eV corresponding to Mo 3d5/2 and Mo 3d3/2
respectively, suggest that the composition of the as-prepared
catalyst is MoO3, which is in accordance with the EDS result.
Compared to the fresh a-MoO3, there are two other small peaks
at 231.2 and 234.4 eV in the Mo 3d spectrum of a-MoO3-air,
indicating that a tiny amount of Mo with a lower oxidation state
(between VI and IV) was formed. Similarly, a higher ratio of
Mod+/Mo6+ (4 o d o 6) is obtained after the reaction performed
under a N2 atmosphere.
From the XPS results, under atmospheric air, the ratio of Oa/
(Oa + Ob) was slightly decreased in the O1s spectra, indicating
that lattice O2À was consumed after the reaction. When the
HMF oxidation reaction was performed under a N2 atmosphere,
the content of lattice O2À decreased more obviously. These
results illustrate that the consumed lattice O2À on the surface
of a-MoO3 can be replenished in the presence of atmospheric
air. However, the recovery rate of active lattice O2À species is
lower that the consumption of them as the ratio of Oa/(Oa + Ob)
Fig. 7 One-step synthesis of DFF from fructose catalyzed by a-MoO3
under atmospheric air. Reaction conditions: fructose 0.18 g (1 mmol), a-
MoO3 50 mg, DMSO 2 mL, 120 1C.
form DFF along with the reduction of Mo6+ to Mod+. Finally, the
Mod+ was reoxidized by the chemisorbed molecular oxygen,
which continuously supplied the active lattice O2À species via
the cyclic redox couple of Mo6+ and Mod+ ions.
on the surface of a-MoO3-air is decreased. Moreover, the One-step synthesis of DFF from fructose catalyzed by a-MoO3
increased content of Ob is indirect proof of the increased under atmospheric air
content of oxygen vacancies, and the Mod+ is relevant to the
From an economical viewpoint, fructose is a more attractive
raw material to prepare DFF via acid-catalyzed dehydration and
selective oxidation by a one-step method. Considering the
excellent acidity and oxidizability of a-MoO3, we attempted
creation of oxygen vacancies. In the Mo 3d XPS spectra, the
appearance of Mod+ on the surface of a-MoO3-air indicates that
more oxygen vacancies appear on a-MoO3-air. The similar
increasing trend in the content of lattice O2À and Mod+ on
the synthesis of DFF directly from fructose under atmospheric
the surface of used a-MoO3 (in air and N2) identifies that the
air. Firstly, fructose was used as the raw material to produce
HMF and DFF in the catalysis of a-MoO3 under different
reaction temperature and time to obtain the highest carbon
balance. The results are shown in Fig. S4 (ESI†), the highest
carbon balance was calculated as 83% which was obtained at
120 1C for 2 h. Then, the reaction time was prolonged to achieve
the highest yield of DFF. As shown in Fig. 7, both HMF and DFF
were detected at the initial reaction time. The intermediate
lattice O2À species is closely associated with the redox potential
of a-MoO3. According to the XPS analysis results, the lattice O2À
species serves as the active sites in the HMF oxidation reaction.
The reaction undergoes the following pathways according to
the Mars–van-Krevelen mechanism. In Scheme 1, the hydro-
xymethyl species of HMF were firstly adsorbed on the surface of
a-MoO3 and then the lattice O2À species were consumed to
product HMF attained a maximum yield of 56.1% in 1 h and
received full conversion in 8 h. The yield of DFF increased
gradually with reaction time, reaching the highest of 78.3% in
8 h. After the reaction, the reaction mixture was heavily
coloured, indicating the formation of humins in the fructose
dehydration step. The selectivity of DFF in the HMF oxidation
step is calculated as 94.3% according to the carbon balance in
2 h (83%) and the yield of DFF in 8 h (78.3%), which is a little
lower than that obtained by using HMF as the raw reactant
(97.2%). The reason for this smay be that a little amount of
HMF or DFF was adsorbed on the surface of the formed
humins.
Conclusions
a-MoO3 nanobelts were successfully synthesized by a simple
hydrothermal method. The characterization results reveal that
Scheme 1 Possible reaction mechanism for a-MoO3-catalyzed oxidation
of HMF to DFF under atmospheric air.
the a-MoO3 nanobelts are well-crystallized single crystallites,
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possess medium acid sites and have good reducibility which
may favor the one-step reaction of fructose to DFF. Then, a-
MoO3 was applied as a bifunctional catalyst for DFF production
under atmospheric air. Under optimum reaction conditions,
high DFF yields of 97.2% and 78.3% were obtained by using
HMF and fructose as reactants, respectively. Furthermore, a
plausible reaction pathway was proposed for a-MoO3-catalyzed
oxidation of HMF to DFF based on the experimental and
catalyst characterization results. The lattice O2À on a-MoO3
serves as the active sites to realize the oxidation of the hydroxy
group in HMF. The consumption of the lattice O2À on a-MoO3
is replenished by the chemisorbed molecular oxygen during the
reaction and then a-MoO3 is involved in the next catalytic cycle
process. Additionally, the a-MoO3 nanobelts exhibited high
stability and could be used at least five times without obvious
loss in their catalytic activity. In brief, a-MoO3 is an easily-
prepared, eco-friendly, low cost and highly efficienct catalyst for
one-step conversion of fructose to DFF under atmospheric air.
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (21805065). We also thank the support from
Anhui Province Key Laboratory of Advanced Catalytic Materials
and Reaction Engineering (PA2020GDSK0064).
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